Pathways by which stimuli originating in the cingulate cortex inhibiting LH secretion reach the hypothalamus

The effect of transecting the frontal, lateral, dorsal or caudal afferents to the mediobasal hypothalamus (MBH) on the inhibition of LH release induced by electrochemical stimulation of the anterior cingulate area was studied. Electrochemical stimulation (100 microA/30 s) of the anterior cingulate area applied in ovariectomized estrogen-primed rats immediately after the first 15 min of electrical stimulation in the medial preoptic area (mPOA; monophasic square pulses of 1 ms duration, 50 Hz, 150 microA, 15 s on and 15 s off for 30 min) suppressed the rise in LH observed in control rats 15 min later. This inhibitory effect was prevented either by bilateral parallel cuts along the lateral hypothalamic area (LHA) or by cuts in the premammillary area (PMA) interrupting the caudal afferents to the MBH, whereas it was not affected by suprachiasmatic or periventricular cuts or by transection of the medial corticohypothalamic tract. Unilateral cuts in the LHA also suppressed the effect of anterior cingulate area stimulation when the cortex of the frontal lobe ipsilateral to the side of the lesion was stimulated. None of these lesions interfered with the release of LH triggered by stimulation of the mPOA. Lesions in the LHA or PMA also prevented the blockade of ovulation and discharge of LH on the day of Proestrus induced by electrochemical stimulation of the anterior cingulate area. Besides, electrolytic lesions of the ascending noradrenergic pathways at the level of the dorsal noradrenergic tract or the medial forebrain bundle but not of the locus ceruleus or the A5 cell grouping also suppressed the inhibition of LH secretion. These results indicate that the impulses originated in the anterior cingulate area inhibiting LH release reached the hypothalamus by way of its lateral and caudal aspects and that an adrenergic mechanism is associated with this effect.